Disc drives are data storage devices used to store and retrieve digital user data in a fast and efficient manner. A typical disc drive stores such data on a number of magnetic recording discs which are rotated at a constant high speed. An actuator controllably moves a corresponding number of data transducing heads to access data stored in tracks defined on the disc surfaces.
Servo data are written to the discs during disc drive manufacturing to define the tracks and to provide head positional information to a closed loop servo control circuit. The servo data are used by the servo control circuit during both seeking and track following operations. In a typical embedded servo scheme, the servo data are arranged in servo sector patterns. The servo sectors are angularly spaced apart and interspersed with user data sectors to which user data are stored.
The servo data include index data used to identify an index reference position on the disc surfaces. The index reference position corresponds to a “once-around” (i.e., zero degrees) angular reference for the discs. A typical servo control circuit tracks the angular position of the head by counting the number of servo fields encountered after each occurrence of the index reference position.
When multiple recording surfaces are used, it is often desirable to provide a selected angular alignment of the servo sectors on the various surfaces. In this way, a single servo burst counter can be used to track the angular position of the discs with respect to the heads (i.e., the number of servo sectors since the most recent index reference position) as different heads are selected in turn.
With the continued demand for disc drives that provide ever higher data storage capacities and transfer rate performance levels at lower costs, designers continue to provide successive generations of products with ever higher areal data storage densities. It will be recognized that for a given area on the recording surface of a disc, more data can be stored by increasing the number of bits per linear distance along the tracks (e.g., bits per inch, BPI) as well as by increasing the number of the tracks per distance across the radius of the disc (e.g., tracks per inch, TPI). Achieving a higher track density generally requires making the physical width of each track narrower. This in turn requires higher levels of precision in the writing of the servo data.
Historically, the servo data have typically been written to the discs in-situ, that is, after the discs have been installed into the drive. In such a system a fully or partially assembled drive is provided to a servo track writer (STW) station. The station employs a precisely controlled positioning arm or other mechanism to advance the actuator across the discs at selected increments to allow the disc drive heads to write the servo data.
More recently, some manufacturers have moved to the use of multiple disc writer (MDW) stations. An MDW station prerecords the servo data onto multiple discs at a time using specially configured, low vibration disc motors and actuators. After the servo data have been prerecorded to the discs, the discs are removed from the MDW station and installed into the drives.
While MDW stations have been found to provide significant improvements in the writing of servo data, excessive angular misalignments in the servo data can sometimes arise when the prerecorded discs are subsequently installed onto a disc drive spindle motor. However, drives of the present generation can use around 200 servo sectors per revolution, meaning that each servo frame window (i.e., the distance between adjacent servo sectors) is a little less than two degrees around the disc circumference. It is difficult using existing manufacturing processes to ensure that all of the servo sectors on multiple disc surfaces will be aligned within the required timing tolerances when prerecorded discs are stacked onto a spindle motor hub. Head-to-head skew (misalignments) within the actuator can further reduce the margin available to achieve the desired servo sector alignment.
Even if the servo data are written using an STW station so that the same heads that write the servo data are also subsequently used to transduce the servo data during operation, excessive misalignment of the servo sectors can still arise. In order to achieve the desired amount of precision in the writing of the data, the positioning arm in a typical STW station typically clamps and holds the actuator in a rigid fashion. It has been found that once the positioning arm releases the actuator, the heads can move to different positions (i.e., head skew is introduced). Thus, even if the servo data are perfectly aligned during the STW operation, the resulting head skew after the STW operation is completed can result in excessive misalignment with respect to the final relative positions of the heads.
Accordingly, with continued demands for ever higher precision in the writing of servo data, there remains a continued need to promote improved angular alignment of servo sectors on different disc recording surfaces in a disc drive. It is to such improvements that the claimed invention is directed.